GEOLOGY – Vol. II - Occurrence, Texture, and Classification of Igneous Rocks - Gezahegn Yirgu
OCCURRENCE, TEXTURE, AND CLASSIFICATION OF
IGNEOUS ROCKS
Gezahegn Yirgu
Addis Ababa University, Ethiopia
Keywords: crystal twinning, crystal zoning, extrusion, felsic, glass, intergrowth,
intrusion, IUGS, lava, mafic, magma, mineral, modal composition (mode), normative
composition (norm), nucleation, plutonic, polarizing microscope, pyroclast, texture, thin
section, viscosity, volcanism
Contents
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1. Introduction
2. Mode of Occurrence of Igneous Rocks
2.1. Lava Flows and Domes
2.2. Pyroclastic Deposits
2.3. Intrusive Bodies
2.3.1. Plutons
2.3.2. Minor Intrusions
2.3.3. Layered Intrusions
3. Texture of Igneous Rocks
3.1. Crystal Nucleation and Growth
3.2. Common Igneous Textures
3.2.1. Crystallinity
3.2.2. Granularity
3.2.3. Crystal Shapes
3.2.4. Mutual Relations of Crystals (and Amorphous Materials)
4. Classification and Nomenclature of Igneous Rocks
4.1. Classification Scheme for Igneous Rocks
4.2. Principal Factors in Classification
4.2.1. Modal Mineralogy
4.2.2. Grain Size
4.2.3. Nomenclature
4.2.4. Chemical Classification
4.3. Classification and Nomenclature of Plutonic Rocks
4.3.1. Ultramafic Rocks
4.3.2. Charnockitic Rocks
4.4. Classification and Nomenclature of Volcanic Rocks
4.5. Classification and Nomenclature of Pyroclastic Rocks
4.6. Other Igneous Rock Groups
Acknowledgements
Glossary
Bibliography
Biographical Sketch
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GEOLOGY – Vol. II - Occurrence, Texture, and Classification of Igneous Rocks - Gezahegn Yirgu
Summary
In this article, the manner in which igneous rocks occur in nature is introduced,
followed by a discussion of the general textural features and appearance of these rocks.
The classification and nomenclature of igneous rocks are treated in the final section.
Igneous rocks are formed when magma cools, either on Earth’s surface or beneath it,
though some may form by fragmentation of solidifying magma. Typical occurrences of
igneous rock bodies on the surface include lava flows, lava domes, necks and spines,
and pyroclastic deposits. Plutonic bodies with different dimensions and geometry form
by cooling of magma beneath the earth surface, and are exposed at the surface by postemplacement geological processes and erosion.
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The term “texture” is used in the conventional geological meaning for features such as
grain shape and size, and spatial relationships between mineral grains in a rock. The
subject is discussed in terms of recent research on the nature of crystallization.
The recommendations of the International Union of Geological Sciences (IUGS)
subcommission for igneous rock terminology and classification are illustrated. For the
sake of completeness, and in order to provide a general picture of the whole spectrum of
igneous rocks, also included are a number of types that are either rare or difficult to
identify without the use of advanced techniques.
1. Introduction
For centuries people living near volcanoes have noticed that the red-hot molten material
that erupted onto Earth’s surface as lava cooled and solidified to give solid rocks. Lava
(from the Italian lavare: to wash) was originally applied to streams of water, and in the
eighteenth century in Neapolitan dialect to streams of molten rock from the Vesuvius
volcano. The term is now used for the molten material that erupts from volcanoes as
well as for the rock that forms on solidification of this material. Rocks resulting from
volcanic eruption represent only a small proportion of those rocks formed by the
cooling and crystallization of magma, most of which occurs at depths beneath the
Earth’s surface.
All rocks represent the final products of a multitude of physical and chemical processes
(see Igneous and Metamorphic Petrology; Processes of Magma Evolution, Magmatic
Suites and Consequences for the Composition of Continental Crust). Throughout their
cooling history all magmatic rocks try to achieve mechanical and chemical equilibrium,
but rarely succeed. As a result, they leave behind trails consisting of wide chemical and
mineralogical variations, disequilibrium mineral assemblages, disequilibrium textures,
fluid inclusions, and so on, which when taken together, permit us to reconstruct the
history of the magmatic rock.
This article attempts to cover the description, mode of occurrence, and classification of
igneous rocks. The article follows a purely descriptive approach, providing factual data
and attaching names to igneous rocks. This is not an end in itself, but rather one step
toward understanding how rock bodies form and how magmatic eruptions behave.
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GEOLOGY – Vol. II - Occurrence, Texture, and Classification of Igneous Rocks - Gezahegn Yirgu
2. Mode of Occurrence of Igneous Rocks
Magmas erupted from volcanoes are either poured out as coherent fluidal lava flows or
blown out as fragments of various sizes. A body of magma may also be emplaced and
cool beneath the surface of the Earth. Igneous rocks result from the final solidification
of magma at the surface or at variable depths within the Earth, as well as from the
eventual consolidation of fragmented debris.
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Igneous rocks thus occur in two ways, either as “extrusive” (on the surface) rocks or as
“intrusive” (below the surface) bodies. Intrusive rocks are also called “plutonic” (Pluto,
the Greek god of infernal regions, therefore deep-seated) and extrusive rocks
“volcanic.” The terms intrusive and extrusive only refer to the place where the rock
solidified. Extrusive rocks cool rapidly because they have erupted at the Earth’s surface,
but intrusive rocks cool more slowly within an insulating blanket of surrounding rocks
into which they have been emplaced. The rapid cooling of magma gives a fine-grained
rock, which may even be glassy, whereas slower cooling gives coarse-grained rock with
large crystals.
2.1. Lava Flows and Domes
In its upward movement, magma may be erupted at the surface from fissures or volcanic
vents. Fundamentally, differences in magma composition and volatile content are
responsible for all variations between the extremes of quiet lava effusion and
catastrophic explosion. Some volcanic eruptions are short and sharp, whereas others
drag on for months through various phases with different eruptive styles.
Effusive activity is dominated by passive emission of “lavas.” Lavas may be emitted
from fissures or central vents. Several central eruptions may line up along a great
fracture or fissure zone.
Lava flows extruded on the earth’s surface range from a few centimeters to a few
hundred meters in thickness. The area may be a few square meters or many square
kilometers. Extrusions display a wide range of forms, depending upon their mobility or
apparent viscosity.
Lava flows are tabular igneous bodies, generally thin compared with their horizontal
extent. The attitude corresponds in a general way to that of the surface upon which they
are erupted. On flat plains, the lava flows are more or less horizontal; but on the slopes
of volcanoes, they may consolidate with a considerable inclination. Relatively lowviscosity or fluid flows (basaltic magmas) spread out from the vent as thin extensive
sheets, whereas viscous rhyolite lavas are thick and short. At the largest scale, there is a
clear separation between effusive and explosive styles by composition. Large-volume
basaltic eruptions are almost exclusively effusive; large-volume silicic eruptions are
exclusively explosive.
In general terms, if a volcano is built up by a single eruption, it is called “monogenetic.”
If there are repeated episodes of activity from the vent, a bigger “polygenetic” volcano
results.
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GEOLOGY – Vol. II - Occurrence, Texture, and Classification of Igneous Rocks - Gezahegn Yirgu
“Fissure” eruptions result when magma-filled dykes intersect the surface. When a dyke
of mafic lava breaks through to the surface, huge-volume basaltic flows may be formed.
In historic times, the largest example of this phenomenon took place between June 1783
and February 1784, when 14 cubic kilometers of lava flooded from the 25 km length of
the Laki Fissure in southern Iceland.
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Central eruption forms various types of volcanic edifices, among which “shield
volcanoes” and “stratovolcanoes” are the largest. Shield volcanoes are broad cones with
low angles of slope, built chiefly of fluid basaltic lava emitted prevailingly from a
central area. Stratovolcanoes are formed by mixed effusive and explosive activity, and
consist of alternating lava flows and pyroclastic deposits. In central volcanoes, variable
amounts of lava and tephra erupt from vents or fissures on the side of the cone. Very
symmetrical volcanic cones (for example, Mayon in the Philippines, or Fuji in Japan)
are built up by lavas and pyroclastic deposits emitted almost exclusively from a central
vent. Other asymmetrical large volcanoes are more complex, and result from eruptions
that took place both from the summit vent, and from various craters along the flanks.
In volcanic complexes resulting from fissure eruptions accompanied by swarms of
feeders, areas of several hundred square kilometers may be flooded by lava flows
amounting to thousands of meters in total thickness. These are known as large igneous
provinces (LIPs). Most LIPs, such as the Deccan Plateau of western India, the Parana of
South America, and the high plateaus of Ethiopia and East Africa, have been
constructed primarily by fissure eruptions. Basalts constitute more than 90 percent of
the lavas participating in the eruptions.
The surface of lava flows is generally covered by irregularities of different magnitude
and shape, which depend on the magma viscosity. Pahoehoe lava has a smooth billowy
or ropy surface, and is typical of very fluid basaltic magmas. Aa lava is typical of
slightly more viscous basaltic magmas and consists of irregular blocks that are
commonly covered with small spines. Blocky lava is composed of irregular blocks that
lack spines, and are typical of intermediate and salic magmas. Subaqueous basaltic
volcanic eruptions have characteristic “pillow” structures. Pillows are elongated,
bulbous, tube or sac-like bodies of lava that form by repeated budding and extrusion of
fluid lava at the fronts of underwater flows.
Very viscous acid lava flows are less mobile and are commonly extruded either as
tongue-like lava flows, or as thick, bulbous masses called “domes,” which pile up over
and around their vents. The ratio of horizontal diameter to thickness of lava domes can
be near unity. Some domes are emplaced as nearly consolidated masses, and are simply
thrust slowly out of the vent as a “spine” without much lateral expansion, like a cork
withdrawn from a wine bottle.
2.2. Pyroclastic Deposits
When magmas are rich in volatiles (especially H2O and CO2), they erupt explosively
and become fragmented. Explosive eruptions and magma fragmentation can vary from a
continuous slow process to the devastating process of gigantic eruptions. Eruptions eject
large varieties of pyroclastic material, including large blobs of magma that are still
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GEOLOGY – Vol. II - Occurrence, Texture, and Classification of Igneous Rocks - Gezahegn Yirgu
molten or plastic, intensely fragmented fine ashes, and blocks of solid rocks. As the
eruption continues, ejected material piles up partly around the vent, and partly at a
considerable distance from the vent, depending on the energy of the explosive eruption
and on the dimensions of pyroclasts.
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Scoria cones are small, rarely more than 200 to 300 meters high, monogenic volcanic
features with a distinctive morphology and side slopes close to 33°. They result from
minor moderately explosive basaltic eruptions. Maar craters are simple circular
depressions excavated into the substrate, surrounded by low rims of ejected debris; they
are commonly filled with water and are manifested as lakes. Maars are formed when
basaltic magmas interact with water at shallow depths. Tuff rings are accumulations of
highly fragmented basaltic tephra, with a characteristic broad and flat circular geometry.
Tuff cones are smaller, steeper versions of tuff rings, composed of similar fragmented
tephra, and have a morphologic resemblance to scoria cones.
2.3. Intrusive Bodies
Intrusive rock bodies, formed by slow crystallization beneath the surface of the earth,
can be seen only where they are uncovered by erosion. The existence of an intrusion
demands a mass of older, solid or semisolid country, wall, or host rock. Intrusions are
described as “concordant” if their contacts are more or less parallel to the structures
(bedding or schistosity) of the intruded host rocks, and “discordant” if the intrusive
body cuts across the host rock structures.
Bodies of intrusive rocks vary greatly in form and extent. The smallest are dikes and
veins a few centimeters wide; the larger masses outcrop continuously over areas
measured in thousands of square kilometers. The most important role in the
emplacement of intrusive bodies is played by gravity. Their mode of intrusion is largely
determined by the difference in density between the magma and its country rocks; other
factors, such as the viscosity of the magma or the stress regime in the country rocks,
have only a modifying influence. Some of the commoner intrusive types, classified
according to form and relation to invaded rocks, are described briefly below.
2.3.1. Plutons
Pluton is a term that embraces all intrusive bodies of igneous rock produced by
solidification of large bodies of magma at depths of several km or even tens of km.
Plutons believed to have been emplaced at shallower levels in the crust (< 10 km) can
usually be seen to be discordant, whereas deeper plutons tend to be concordant with the
structure of their country rock. Several types of plutons have been identified on the
basis of their size and geometry (Figure 1).
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Laccoliths (Greek: lakkos, a cistern; lithos, stone) are sheet-like bodies with a
flat base and domed roof, above which the invaded strata have been arched
concordantly at the time of intrusion.
Phacoliths (meaning lens rocks) are curved, lensoid masses injected along and
concordant with the arches and troughs of folded strata. They may be described
as saddle-shaped laccoliths.
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GEOLOGY – Vol. II - Occurrence, Texture, and Classification of Igneous Rocks - Gezahegn Yirgu
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Lopoliths (Greek: lopos, basin) are roughly sheet-like or funnel-shaped bodies
with upper and lower surfaces that are concave upwards, the general
configuration being connected with sagging of the floor rocks under the load of
the thickening intrusions.
Batholiths (Greek: bathos, depth) are intrusions with surface outcrop areas in
excess of 100 km2; they are discordant, usually composite, with steeply
inclined walls and without any visible floor. They are very large features, the
size of mountain ranges, and are typically composed of felsic rocks (granite,
granodiorite, and related rocks).
Stocks: These are similar in form and composition to batholiths but are smaller
in size (with outcrops less than about 100 km2.).
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2.3.2. Minor Intrusions
Figure 1. Cartoon showing occurrence of major types of intrusive body
A minor intrusion is any intrusion too small to be classified as a pluton. Many of these
are emplaced at shallow depth, within a few kilometers of the surface, and very often
reveal links between plutonic and volcanic activity. A number of types are distinguished
on the basis of their geometrical form.
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Dikes: These are tabular, often vertical or steeply inclined sheets that cut across
the trend of structure (for example, bedding planes, cleavage planes) of the
invaded rocks. Dikes tend to occur in swarms, in parallel mode or radiating
from a common center.
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GEOLOGY – Vol. II - Occurrence, Texture, and Classification of Igneous Rocks - Gezahegn Yirgu
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Ring dikes: These are steeply inclined dikes of arcuate outcrop, formed by the
uprise of magma along major steeply conical or cylindrical fractures bounding
central collapsed blocks.
Plugs or necks: These are steep pipe-like bodies with relatively small crosssections, less than 100 m in diameter; many are conduits of volcanic vents.
Sills: These are generally horizontal tabular or sheet-like intrusions that are
mostly concordant with and thus injected along the major structure (for
example, bedding or foliation) of the invaded rocks. They can measure
hundreds of meters in thickness and extend laterally for many kilometers.
2.3.3. Layered Intrusions
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These are usually large plutonic rock bodies characterized by prominent phase layering.
The name is traditionally applied to mafic and ultramafic intrusions. These bodies are
commonly funnel-shaped, and characteristically show pervasive mineral layering that is
discordant with the walls of the funnel.
Single layers range from millimeters to hundreds of meters in thickness, and from
meters to tens of kilometers in lateral extent.
Layering is most conspicuously defined by variations in relative proportions of minerals.
Gradational variations within a single layer, from top to bottom, may be obvious.
Sorting of minerals by mineralogy or by grain size may show layering. Other types of
layers show no sorting and are essentially isomodal throughout, standing out by virtue
of their contrast with adjacent layers.
The rhythmic alternation of sorted or isomodal layers is the most visible aspect of
layered intrusions. The Bushveld complex in South Africa is a typical layered intrusion,
exposed over an area of 65 000 km2 with a thickness of up to 7 km.
3. Texture of Igneous Rocks
By “texture” is meant the general physical appearance of a rock, especially the size,
shape, and arrangement of constituent mineral grains. It is generally defined as the
geometrical relationship among the component minerals of a rock and any amorphous
materials (glass or gas in cavities) that may be present.
Understanding igneous textures involves understanding crystals; what they are, how
they form and what controls their numbers, shapes, and sizes. Thus crystals may be
euhedral, subhedral, or anhedral; they may be relatively large or small, and they may
exhibit a great diversity of form ranging from regular forms with planar faces to skeletal
crystals, and from highly inequidimensional to equant forms.
In addition, crystals may be twinned or zoned, and one mineral may be intergrown with
another. The reason for such diversity is in part found in the variety of circumstances
that lead to crystallization, including the cooling of magma, devitrification of glass,
production of new stable phases concomitant with the breakdown of unstable material,
and precipitation from solution.
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Bibliography
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Best M. G. and Christiansen E.H. (2001). Igneous Petrology. 458pp.Malden, MA: Blackwell Science.
[This is a textbook that provides comprehensive coverage of physical and chemical aspects of magmatic
rocks and dynamic magma systems.]
Carmichael I.S.E., Turner F.J., and Verhoogen J. (1974). Igneous Petrology. 739pp. New York: McGrawHill. [This is a widely used textbook that presents essential principles on the nature, origin, and evolution
of igneous rocks.]
Cas R.A.F. and Wright J.V. (1987). Volcanic Successions: Modern and Ancient. 528pp. London: Allen
and Unwin. [This is a book that deals with the broad spectrum of volcanic facies types, the processes
responsible for their formation, and their significance in terms of depositional setting.]
Cox K.G., Bell J.D., and Pankhurst R.J. (1979). The Interpretation of Igneous Rocks. 450pp. London:
Allen and Unwin. [This book deals with how observational features and analytical data of igneous rocks
can be used to characterize and classify igneous rocks as well as unravel their sources and evolutionary
histories.]
Francis P. (1993). Volcanoes: A Planetary Perspective,.443pp. New York: Oxford University Press. [This
book presents a detailed account of the familiar violent aspects of volcanoes and the various forms that
eruptions can take, with examples of major historical eruptions.]
Le Bas M.J., Le Maitre R.W., Streckeisen A., and Zanettin B. (1986). A chemical classification of
volcanic rocks based on total alkali–silica diagram. J. Petrol., 27, 745–750. [This paper describes the
classification and nomenclature of volcanic rocks based on major element chemical data as proposed by
IUGS Sub-commission on the Systematics of Igneous Rocks.]
Le Maitre R.W. (ed.), Bateman P., Dudek A., Keller J., Lameyre J., Le Bas M.J., Sabine P.A., Schimd R.,
Sorenson H., Streckeisen A., Wooly A.R., and Zanettin B. (1989). A Classification of Igneous Rocks and
Glossary of Terms: Recommendations of the International Union of Geological Sciences Sub-commission
on the Systematics of Igneous Rock., 193. Oxford: Blackwell Scientific Publications.
MacKenzie W.S., Donaldson C.H., and Guilford C. (1982). Atlas of Igneous Rocks and Their
Textures.148pp. London: Longman Scientific & Technical. [This is a popular full-color handbook
containing descriptions and photographic illustrations of common textures and appearances of igneous
rocks in thin section under the polarizing microscope.]
Shelley D. (1993). Igneous and Metamorphic Rocks Under the Microscope: Classification, Textures,
Microstructures, and Mineral Preferred-Orientations, 445pp. London: Chapman & Hall. [This is a
modern textbook that provides a thorough integrated treatment of rock characteristics using the polarizing
microscope as the central analytical tool. The central theme is on nature and origin of textures and rock
fabrics.]
Biographical Sketch
Dr. Gezahegn Yirgu, born in Addis Ababa in 1951, is currently Associate Professor of Igneous
Petrology and Geochemistry at the Department of Earth Sciences, Addis Ababa University. He obtained
his B.Sc. in 1977, his M.Sc. in 1980 in Geology from Addis Ababa University, and his Ph.D. in Igneous
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GEOLOGY – Vol. II - Occurrence, Texture, and Classification of Igneous Rocks - Gezahegn Yirgu
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Petrology in 1990 from the University of Pisa, Italy. He has undertaken extensive research in Ethiopia on
flood and rift volcanics, rift tectonics, volcanic hazard, and volcanogenic mineral resources. He has also
served as Dean of the Faculty of Science, AAU, and is Associate Editor of SINET: Ethiopian Journal of
Science and President of the Ethiopian Geoscience and Mineral Engineering Association.
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